In general, the cold rolled steel sheets for press forming which are used for outer plates of automobiles, gasoline tank and the like are required to be excellent in stretch formability, deep-drawability and aging resistance.
The lower the yield strength (YS) and the higher the elongation (EL) and the work hardening exponent (n value) as the material properties, the more excellent the stretch formability of the steel sheets.
The deep-drawability in the material properties is almost dominated by the Lankford value (r value). The higher the r value, the higher the limit of the deep-drawability.
On the other hand, it has been known that when there remain C,N in a solid solution state in the steel sheet, the trouble called "stretcher strain" occurs upon working of press forming due to aging at room temperature. This necessitates the aging resistance, which is ordinarily evaluated by using the aging index (AI). This index is represented by the difference between the yield strength at 7.5% preliminary strain and the yield strength after heat treatment of 100.degree. C..times.30 minutes. The steel sheets for use in press working are required to have the AI value of not more than 3 kg/mm.sup.2.
There have been heretofore proposed many processes of manufacturing the cold rolled steel sheets which are excellent in the above described stretch formability, deep-drawability, and aging resistance. For instance, there is a method of box annealing a low-carbon aluminum-killed steel having a carbon content of about 0.04% by weight (the amounts of the steel ingredients being hereinafter referred to briefly as "%"); and a method of box annealing or continuously annealing a steel sheet in which a carbonitride-forming element such as Ti, Nb or the like is added into the extremely low-carbon steel having a carbon content of not more than 0.1%.
However, these conventional processes have the common feature that the temperature (hereinafter referred to as "soaking temperature") at which steel slabs are uniformly heated prior to the hot rolling is extremely high near 1,200.degree. C.
The reason why the soaking temperature is so high is as follows: In the case of the low-carbon aluminum-killed steels, it is necessary to completely solid-solve AlN when soaking the steel slabs in order to obtain a high r value by the action of AlN precipitated upon box annealing after the cold rolling. Meanwhile, in the case of the extremely low-carbon steel to which are added Ti or Nb, since the Ar.sub.3 transformation point at which the austenite phase is transformed into the ferrite phase is extremely high near 900.degree. C., the hot roll-finishing temperature (FDT) must be high so as to avoid deterioration of the material properties due to the hot rolling at a temperature lower than the Ar.sub.3 transformation temperature.
But, for heating the steel slab at a high temperature of about 1,200.degree. C., a huge energy is not only required but also the higher soaking temperature decreases the yield of the steel slab (due to the surface oxidation), and further promotes the interior oxidization in the vicinity of the surface of the steel slab, so that such a method has the drawback that the trouble such as the surface defect, the surface hardening and the like frequently occur.
As mentioned above, the heating of the steel slab at high temperature leads to not only the consumption of much energy but also the surface defect and therefore there is strongly desired to establish the process of manufacturing the cold rolled steel sheets which lowers the soaking temperature of the steel slab and gives the excellent press-formability.
There have been proposed several processes for manufacturing the cold rolled steel in which the soaking is carried out at a low temperature of not higher than 1,200.degree. C., followed by hot rolling, for instance, Japanese Patent Laid Open Application No. Sho 49-129,622 (Japanese Patent Application No. Sho 48-43,856), Japanese Patent Laid Open Application No. Sho 51-59,008 (Japanese Patent Application No. Sho 49-132,622) and Japanese Patent Laid Open Application No. Sho 55-58,333 (Japanese Patent Application No. Sho 53-129,071). However, in any case, in order to make the hot roll-finishing temperature to be not lower than the Ar.sub.3 transformation point, the soaking temperature must be actually not lower than 1,100.degree. C. and in the very recent Japanese Patent Laid Open Application No. Sho 57-13,123 (Japanese Patent Application No. Sho 55-84,696), the soaking temperature of the steel slab is 1,100.degree.-1,250.degree. C.
To the contrary, in the low-temperature soaking in which the lower limit is 1,100.degree. C., the above described effects for saving energy and avoiding the decrease in the yield are suppressed to an extremely small degree and the material properties of the cold rolled steel sheets are not sufficiently improved as described hereinafter.
In addition to the above, Japanese Patent Laid Open Application No. Sho 53-64,616 (Japanese Patent Application No. Sho 51-140,532) discloses a process of manufacturing a steel sheet having an r value of 1.17-1.20 in which a rimmed steel slab having C of 0.05-0.11% is soaked at 980.degree.-1,050.degree. C., and finished at a temperature of 710.degree.-750.degree. C. Japanese Patent Laid Open Application No. Sho 56-15,882 (Japanese Patent Application No. Sho 55-60,713) discloses a process of manufacturing a steel sheet having an r value of 1.1 in which a steel slab having C of 0.03% and Al of 0.05% is soaked at 950.degree. C. and finished at a temperature of 750.degree. C. However, they both relate to the manufacture of the steel sheets having an r value being as low as not more than 1.2 and essentially differ from the deep-drawing steel sheet aimed at by the invention.